Olefin metathesis reactions (also referred to as dismutation or disproportionation reactions) wherein an olefin is converted to a product having a higher or lower carbon number than the starting material are known. These reactions are of considerable interest because of the versatility of the reaction and the numerous olefinic hydrocarbons available from petrochemical sources which are suitable for use in the reaction to yield useful products. While both heterogeneous and homogeneous catalysts can be used for these reactions the heterogeneous catalysts generally require higher reaction temperatures and pressures and give lower selectivity, particularly when using higher olefins. For these reasons recent emphasis has been to the development of new and improved homogeneous catalyst systems.
Homogeneous catalysts presently known and available are largely based on tungsten and molybdenum compounds, such as tungsten hexachloride, molybdenum pentachloride or tungsten oxytetrachloride, in combination with an organometallic compound or Group 1a, 3a or 4a of the Periodic Table (Handbook of Chemistry and Physics, 56th Edition, 1975-76) most notably lithium, aluminum, germanium or tin. For example, British patent specification No. 1208068 discloses a homogeneous disproportionation catalyst system of molybdenum pentachloride or tungsten hexachloride and an organometallic compound of germanium or tin. The preferred catalyst for the process is derived from tungsten hexachloride and tetra-n-butyl tin. A binary catalyst system of tungsten hexachloride and n-butyllithium is also described by J. Wang et al. in the Journal of Organic Chemistry, Vol. 33, No. 10 (1968) at pages 3794-6. A tertiary catalyst system consisting of a tungsten or molybdenum salt, an organotin compound and a boron halide or its etherate is also disclosed in U.S. Pat. No. 3,901,866. The catalysts are employed to polymerize cyclopentene to obtain cis- and trans-polypentenamers. The disproportionation of olefins in the homogeneous phase employing a catalyst consisting of an alcoholate of molybdenum or tungsten and an organometallic reducing agent is described in U.S. Pat. No. 3,855,340.
V. M. Kothari et al. in the Journal of Organic Chemistry, Vol. 35, No. 20 (1971) at pages 2951-53 report that triphenylphosphine hinders the metathesis of 2-pentene using a tungsten hexachloride/ethylaluminum dichloride cocatalyst system. It has also been reported by K. Ichikawa et al. in the Journal of Organic Chemistry, Vol. 41, No. 15 (1976) at pages 2633-35, that tri-n-butylphosphine is not an effective additive for the tungsten hexachloride/tetrabutyl tin catalyzed metathesis of 1-octene. Whereas esters, acetronitrile, phenylacetylene, dicyclopentadiene and ethers were observed to increase the selectivity of the metathesis by depressing side reactions, no such improvement was obtained when tri-n-butylphosphine was employed as the additive.
It is possible with the known homogeneous catalyst systems to obtain high product selectivity, however, this is typically accompanied by low conversion of the olefin. When measures are taken to increase conversion there is typically a corresponding decrease in the product selectivity. Also, with the previously reported tungsten/organotin cocatalysts there is considerable inconsistency in the results obtained so that even when optimum molar ratios and reaction conditions are employed drastically different conversion and product selectivity can be obtained from run to run. These variations have generally been attributed to indefinable variations in the tungsten compound. For example, tungsten compounds obtained from different suppliers and manufactured to the same product specifications can give markedly different results. Also, variations are often noted with tungsten reagents which have been stored even for a short period of time following all of the recommended storage procedures. Especially in the case of tungsten hexachloride, widely divergent results are obtained with reagents obtained from different suppliers even though the products are, by all measurable standards, identical or when the reagent has been stored for a period of time even though all the prescribed storage precautions have been strictly adhered to. In practice it has not been possible to obtain consistently high conversion of .alpha.-olefins with high selectivity to the desired product using heretofore known tungsten/organotin homogeneous catalyst systems.
It would be extremely advantageous therefore if it were possible to consistently obtain increased conversions with high product selectivity in .alpha.-olefin metathesis reactions employing a homogeneous catalyst system. It would be even more desirable if such metathesis reactions consistently gave high conversions with greater than 95% selectivity and if the catalysts could be obtained by simple modification of a tungsten/organotin cocatalyst with readily available and economical modifying agents. It would be even more advantageous if the improved catalysts and results were obtained without regard to the source and storage history of the tungsten compound. These and other desirable features are now fully realized with the modified homogeneous olefin metathesis catalysts which will be described more fully below.